The focus is the operation that provides a clear subject image as a whole and especially the part of the subject is the focus. According to the lens focal length in relation to any subject, we observe various behaviors of the projected image on the film or sensor.

The focus in a view purely technical is simply a clever compromise between the focal length of the lens used and the subject distance from the lens. No surprise that the focus ring a lens is expressed in the distances form (distance to the subject !). Depending on the subject distance to make a fair compared to the photographer, the focus ring can be adjusted to achieve sharpness by matching the distance value of this ring on the lens with the real effective subject distance of interest.

Autofocus is a technology which assist the photographer in estimating the distance and automatically report on the ring on the lens instantly to ensure clarity on the point selected by the operator via a manually selected point or automatically.

Unresolved focus (left) ; Set focus (right)

Manual focus

Autofocus to image contrast measurement

Autofocus phase contrast detection

Born in 1982, I am part of the last generation that experienced the manual focus on film before the generalization of AF. However, even today, in the achievement context of very specific photographs, manual focus is still a feasible option and accessible at all times, even on the latest digital compact all available on the market.

To facilitate the focus, one has to engineer originally from the School of Fine Arts named Lucien Dodin, the invention of the stigmometer in 1943 that he preferred to call "cut rangefinder field" or "rangefinder Dodin". The stigmometer belongs to the large family of rangefinders and specifically telemetry coincidence lines. Incorporated in standard focusing screen many analog cameras before the autofocus advent, it allowed a simple and precise principle to focusing on any topic. It curiously recalls the Foucault knife principle. This spot divides the image into two. If the point of sharpness was not confused with the film plane, the image appeared clearly divided. To develop the image was to act on the lens until the image appears uniform, without division.

Autofocus has eliminated the dreaded stigmometer on all SLR cameras since the early 90's. This is an electronic eye simplifying focus on bringing much responsiveness to capture very fast scenes. Welcome to the electronic telemetry with two passive systems. These systems do not require signal transmission. This passive telemetry operates only light reflected from the subject.

The electronic telemetry team image contrast measurement mainly compact cameras. Less expensive, it is also less efficient than SLR based on the phase contrast detection because this measure is the exploitation of the image main sensor to perform the focus. Very random performance of this autofocus type in difficult conditions are explained by the nature of his considering procedure in the image sharpness proportional to the contrast intensity. The image contrast measure consists in comparing several successive images produced by the main sensor.

The principle of this measure makes the autofocus exceedingly swift little agitated on a subject whose distance changes very frequently between two successive measurements.

The DSLR autofocus uses an electronic telemetry principle very different from that of a compact. Phase contrast detection is the automated version of the telemetry coincidence lines. Several sensors are coordinated leading to a very similar operation in the Lucien Dodin stigmometer. The autofocus principle is look the same image in two ways opposed to the lens center and measure the symmetrical offset contrast fringes of the two images as a lens to cut field.

Unlike autofocus to image contrast measurement, that of a DSLR does not search point but the calculation only once from wherever symmetrical shift between the two images and control the lens for cancel the difference. This results in a much higher reactivity to that of a compact as the unit will search any sharpness from multiple images as a image contrast measure. The time savings and accuracy are significant, all the more so with the simultaneous use of very silent supersonic motor installed in the AF. Microprocessors built by NIKON, CANON, SIGMA TAMRON.

(Auto)focus

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OBSERVATION AND IMAGERY

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Arnaud FIOCRET
 

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